This invention relates to oscillator circuits and, in particular, to balanced oscillator circuits having spurious free output signals.
Balanced oscillators have many applications and are widely used in the art. A typical balanced oscillators in a circuit that provides signals of a predetermined frequency at first and second outputs, whereby the output signal at the first output is substantially 180 degrees out of phase with respect to the output signal at the second output.
One type of a balanced oscillator circuit is a common emitter oscillator circuit which comprises first and second transistors the emitters of which are coupled together. Further, the base of the first transistor is coupled to the collector of the second transistor while the base of the second transistor is coupled to the collector of the first transistor. Also, a parallel inductive and capacitive tank circuit is coupled between the collectors of the first and second transistors. The common emitter oscillator circuit generates an output signal of predetermined frequency based upon the values of the external inductor and capacitors used in the tank circuit. However, due to the internal parasitic capacitors of the first and second transistors, the common emitter oscillator circuit typically produces undesired output signals at various frequencies wherein these undesired output signals are commonly referred to as "spurious" signals and exist in addition to the desired output signal of predetermined frequency.
Another type of balanced oscillator is a common collector oscillator circuit which comprises first and second transistors such that their collectors are coupled together. Further, an inductive-capacitive tank circuit is typically coupled between the base and emitter of the first transistor and the base and emitter of the second transistor. However, as with the common emitter oscillator circuit, the internal parasitic capacitors of the first and second transistors can generate undesired spurious signals.
Hence a need exists for a spurious free balanced oscillator circuit for providing an output signal of predetermined frequency.